JEREMY IRVINEAlbert, who joins up to bring his horse, Joey, back from France

Sitting in his Los Angeles hotel, Irvine says he's "living the dream". But does he really mean that? The 21-year-old is the lead in War Horse and, after talking to me, is flying back to London to finish filming his role of Pip in Mike Newell's adaptation of Great Expectations. Surely not even his wildest dreams could have made room for this?

"I was pretty desperate to get noticed," he admits. "After one year at drama school, I traipsed round Soho knocking on agents' doors and popping DVDs of my work through letter boxes. But I hadn't actually done any work – I'd got together with a mate of mine who's a great cameraman and we'd shoot scenes that looked like they were out of professional productions and I cut them all together into a show reel."

Even when he was called for an audition for something called Dartmoor, he had no idea what it would entail until the brief script arrived. "I just recognised this scene of a boy talking to his horse as something I'd heard before," he says.

"I loved Michael Morpurgo's book when my parents read it to me when I was seven or eight, but I hadn't seen the play. Somehow I got a couple of tickets for it on the Friday night before I was due to go to the audition on the Monday, and I think that was really important."

It's almost pointless asking him how Spielberg is as a director, given that he has nothing else to compare him with. "On set, he often films the very first take, no rehearsal, and that demands you turn up and be as truthful and as present in the moment as you can possibly be. And when Spielberg's in charge, that seems to be very easy for everyone to do, not just me, but all the others were saying it too."

Spielberg famously sits in a director's chair with DAD written on the back. Is he really as paternal as all that? "He's more like the best teacher you've ever had. But he's also great at feigning innocence himself, so each shot is as if it's his first.

"Only after the day's shooting he'll suddenly start talking about how such and such a shot reminds him of something they did on Jaws, or he'll go into an anecdote about making ET, and you suddenly realise, my God, this is Spielberg I'm working with."

MATT MILNEAndrew, the young farm hand who is Albert's best friend

"I was plucked out of nothing really," is Milne's reply to how he ended up in War Horse. He had not completed his undergraduate drama studies at the University of East Anglia and needed permission for time off to finish filming.

"I figured most of the best actors around 20 years old would all be at drama school, where they're tied up and contractually can't work, so I saw there must be a gap in the market for a young actor like me," he says.

Hereford-born Milne, however, thanks his father for landing him the job of Andrew, best friend to Albert, in War Horse. "Dad's a gardener by trade and works with a lot of farmers so he gave me loads of useful material for my improv audition the night before I went down to London," recalls the 21-year-old, who knew little more than that he was up for the part of a young farm hand. "He told me about lifting up sheep to shear them and at the same time scraping maggots out of their hooves and I just riffed a character on all that – apparently Spielberg loved it." However, when casting director Jina Jay told him: "Steven loved your tape," Matt insists he asked: "Steven who?"

ROBERT EMMSDavid Lyons, Albert's Devon rival for the local girls

Robert Emms will be familiar to War Horse fans who saw him play the lead in the acclaimed West End production. Although he did not land the main role in the screen adaptation, he was happy to be involved again, especially because he had never planned to have a major film career.

"I just wanted to be one of those actors who works at the National Theatre the whole time," he has said. The part of David Lyons was specially created for him, a composite of one character from the novel and another from the stage play. Emms, 25, studied at the Brit School of Performing Arts followed by Lamda, before joining the National Theatre. He was originally cast in a supporting role in War Horse, but took on the role of Albert after the play transferred to the West End.

Despite his initial reluctance about screen acting, Emms has added a couple more major Hollywood films to his CV. He recently appeared as Elizabethan dramatist Thomas Dekker in Roland Emmerich's Anonymous, and this year stars alongside Julia Roberts in Mirror Mirror, an adaptation of the Brothers Grimm's Snow White.

PATRICK KENNEDYLieutenant Waverly, the lighthearted cavalry officer

Benedict Cumberbatch, Tom Hiddleston and Patrick Kennedy play a contrasting trio of friends who serve together as British officers. Cumberbatch is the careerist soldier, Hiddleston the sensitive one, while Kennedy, 34, provides the light relief.

"He is the joker in the pack. I don't think he's a particularly serious soldier, but he's very well meaning," says Kennedy, best known for his role as Carey Mulligan's hapless suitor in BBC's Bleak House. Cumberbatch and Kennedy have a long history together. They were contemporaries at Harrow, starring together in a school production of Death of a Salesman.

Later they both appeared in Atonement, which pleased Spielberg. "My God, you guys are like a club. You're so unbelievably perfect for this, it's great," he told them.

Kennedy had worked with Spielberg before on Munich, but he was delighted to get the nod for War Horse. "I went completely gaga. I got the call at 2am, three months after I put my stuff on tape. I'd completely forgotten about it. Then I went out riding the next day on the beach and broke my arm."

TOM HIDDLESTONCaptain Nicholls, the officer who buys Joey and takes him to war

In the screenplay Nicholls is described as a "handsome, modest, upper-class man" when he first appears knowledgeably inspecting Joey at the livestock auction. With that as a starting point, Hiddleston read Siegfried Sassoon's first volume of autobiography to research the life of a gentleman soldier at the start of the 20th century.

"These guys were amateurs, they knew how to ride but weren't professional soldiers in the way we understand now," he says. "They were innocent and ignorant, confronted by the new technology used by the Germans. It was men with swords against machine guns."

Hiddleston, 30, had ridden "inexpertly" on "big old western saddles" in America, so had to get used to a different style of riding. "There is nothing like the feeling of galloping at 40mph on a living being." His improved horsemanship came in handy in BBC's forthcoming Henry V: "It's nice when the director asks me to gallop up that hill and I can just go off and do it." Last year, he displayed his versatility as the villain Loki in Kenneth Branagh's Thor and F Scott Fitzgerald in Woody Allen's Midnight in Paris.

BENEDICT CUMBERBATCHMajor Stewart, the dedicated cavalry officer

An extraordinary run continues for Cumberbatch. In addition to establishing himself as a modern Sherlock to be reckoned with in the hit BBC TV series, he was also a key member of the lauded cast of last year's Tinker Tailor Soldier Spy movie.

"To go to work every day with such high-calibre actors was a joy," he says. "I had to pinch myself, raise my game and get on with it. And when I got a call to meet Steven Spielberg, it's just the same – stiffen the sinews and be on your mettle, though he's such a relaxed man that you feel very cared for."

Cumberbatch, 35, has since returned to the trenches for a five-part series based on Ford Madox Ford's Quartet, written for the screen by Tom Stoppard and co-starring Rebecca Hall.

"I nearly got blown up by a device the other week," he says. "With War Horse, and now this, I've spent quite a lot of time immersed in the Great War recently, but you never quite get used to imagining how it must have been for men my age 100 years or so ago, thrown into this horror. It's a duty as an actor to respect their memory in a way, and you do feel an almost patriotic pressure to get it true and right."

DAVID KROSSGunther, the resourceful German soldier who befriends Joey

The German actor was still at school when Stephen Daldry cast him as Kate Winslet's 15-year-old lover in The Reader. It was a challenging role for the newcomer because it was his first English-speaking role and it involved full-frontal nudity in the sex scenes. The Reader was only his sixth movie and launched his career internationally: he won the Chopard trophy for the most promising male actor at the 2009 Cannes film festival.

After the success of his breakthrough movie, Kross, 21, enrolled at Lamda, planning to hone his craft and perfect his English. He never finished the course, but who needs formal training when Spielberg offers you a pivotal role in his latest epic?

Like Albert, Gunther is a working-class boy transfixed by Joey when they meet after the opening battle in the film. He is a young German soldier who instantly forges a strong bond with the thoroughbred. "Gunther is a farm boy who has only ever dealt with working horses," Kross has said. "So when he sees this Joey, who's so beautiful and strong, I think it's like seeing a Ferrari out there."

TOBY KEBBELLThe Geordie soldier who risks his life to save Joey

Tipped for stardom since playing the young brother in Shane Meadows's 2004 British drama Dead Man's Shoes, Kebbell is revelling in the limelight. "I don't miss home at all," he says, looking out over Los Angeles. "I've been very busy working, and to be honest I can't say I've been homesick for a second."

But the 29-year-old, who grew up in Nottingham, believes there are striking similarities between the small budget drama that gave him his break and War Horse, in which he plays a significant role as a Geordie officer. "Working with Shane Meadows is just like working with Spielberg. You wouldn't think it on the surface, but they've both got the humility that great men need."

His role in War Horse also revived memories of his theatrical break as part of the 2003 London production of RC Sherriff's Journey's End, also set in the Great War. Although Spielberg is a regular attendee at London theatrical productions, he didn't spot Toby in that particular play. "I think he saw me doing an interview for a Nic Cage movie I did called The Sorcerer's Apprentice. You never know who's watching you and when in this business."

But the realism of Spielberg's reconstruction, he says, will haunt him forever. "Having been in Journey's End, I'd thought about those poor young lads of my age slain on the battlefields, but the theatre could never do what they did here on War Horse," he says. "We had our trenches and when you poked your head above them, three acres of no man's land stretched out. You could see the bomb pits, barbed wire and the dummy bodies. It was an easy job of imagination to tap into my character from there."